This invention relates to micro-electromechanical devices, and more particularly to the optical planarity of micro-electromechanical device gratings.
Micro-electromechanical spatial light modulators with a variety of designs have been used in applications such as display optical processing, printing, optical data storage and spectroscopy. These modulators produce spatial variations in the phase and/or amplitude of an incident light beam using arrays of individually addressable devices.
Chemical mechanical planarization (CMP) has become a key technology as currently practiced in the semiconductor art, for the planarization of metals and dielectrics. In micromachining, the same technique can be used on a fill layer to obtain flat surfaces. However, many of the micromachined structures typically fall into the regime of wide ( greater than 10 xcexcm wide) recesses and sparsely populated structures. One of the difficulties encountered with CMP planarization is the xe2x80x9cdishingxe2x80x9d effect which occurs in the planarization of wide recesses. The xe2x80x9cdishingxe2x80x9d effect during planarization results in thinning of a fill layer in wide recesses and a non-planar surface. The polish rate is affected by the topology of the surrounding areas with dishing becoming worse in sparsely populated regions. Therefore, dishing problems present a severe manufacturing constraint in micromachining.
Non-uniform removal of a fill material across the wafer is also an important consideration in micromachining. When a fill layer is a sacrificial layer, it must be removed outside of the active regions in order to assure adhesion of the release layers. Any residual sacrificial material outside of the active region will be attacked during release. Conventional polishing that ensures complete removal of a sacrificial layer outside of the active region will cause over-polishing and excess removal of the sacrificial material in the active regions.
The dishing phenomenon is illustrated by reference to the schematic cross-sectional diagrams of FIG. 1a and FIG. 1b. Shown in FIG. 1a, is a substrate 100 onto which a first layer 150 is deposited. A narrow recess 110 and the wide recess 120 are shown formed in the first layer 150. The surface of the first layer 150 will contain small areas 130 between recesses and large areas 140 between recesses 110 and 120. Deposited over the first layer 150 and into both the narrow recess 110 and the wide recess 120 is a blanket conformal fill layer 160. Shown in FIG. 1b are the results of planarizing through a conventional chemical mechanical planarization(CMP) method and the blanket conformal fill layer 160 as illustrated in FIG. 1a. As shown in FIG. 1b, the surface of the planarized filled wide recess 170 is severely dished in comparison with the surface of planarized filled narrow recess 180. This marked contrast most resembles the large differences in the problems addressed by the semi-conductor industry versus those skilled in micro-electromechanical systems. Planarized filled narrow recess 180 has the narrow dishing experience in the semi-conductor industry, while planarized wide recess 170 has the complications experienced by the MEMS skilled artisans. A self-aligned mask formed by CMP and used within the severely dished planarized wide recess 170 would be completely polished away in any attempt to address the dishing phenomenon.
There is also shown in FIG. 1b the presence of a fill residue layer 190, formed simultaneously over the small areas 130 and large areas 140 on the surface of the first layer 150 when the blanket conformal fill layer 160 is planarized through the chemical mechanical planarization (CMP) method to form the planarized filled recesses 180 and 170. As is understood by a person skilled in the art, when planarizing large areas of the blanket conformal fill layer 160, generally of dimensions greater than about 1000 microns, the blanket conformal fill layer 160 will in addition to planarizing more rapidly over the wide recess 120 and forming a dish within the planarized filled wide recess 170, simultaneously also polish more slowly over the large area 140 on the surface of the first layer 150 and leave the fill residue layer 190 formed over the large area 140 on the first layer 150. Attempts to remove the fill residue layer 190 by further planarization will cause increased dishing of the planarized filled recesses 180 and 170. Fill residue layers such as the fill residue layer 190 are undesirable since they impede further device processing on the planarized surface. Fill residue layers also impede ribbon attachment to end supports in electromechanical grating structures.
What is needed is a method to create an optically planar surface on the fill layer while eliminating any fill residue layers.
The need is met according to the present invention by providing a method for producing optically planar surfaces for micro-electromechanical system devices (MEMS), comprising the steps of: depositing a first layer over a substrate; forming a channel in the first layer wherein the channel has a depth defined by a thickness of the first layer and a width greater than 10 microns; depositing a second layer over the first layer wherein the second layer has a thickness greater than the depth of the channel and is composed of a different material than the first layer; removing the second layer from outside the channel leaving an overlap at the edge of the channel; and polishing the second layer that fills the channel to obtain an optically planar surface for the MEMS device.
The present invention achieves technical advantages by intentionally removing the second layer outside of the active regions prior to chemical mechanical polishing.